1. Field of the Invention
The present invention relates to a suspension control device which suppresses resonance due to sprung vibration of a suspension of an automobile.
2. Description of the Prior Art
Generally, a vibration model of an automobile as shown in FIG. 8 which is an equivalent model is represented by a vibration having two degrees of freedom such that a mass m1 of a vehicle body (sprung member) is supported by a spring (spring constant: k1) and a damper (damping coefficient: c1) as suspension means, and a mass m2 of an unsprung member such as a wheel is supported by a spring (tire) (spring constant: k2).
In such a vibration model, the vibration of the sprung member due to stimulus from a road surface has two resonance points as shown in FIG. 9. One of the two resonance points is a sprung resonance point due to resonance of the sprung member having a frequency of about 1.5-2 Hz, while the other point is an unsprung resonance point due to resonance of the unsprung member having a frequency of about 10-13 Hz.
As apparent from FIG. 9, it is appreciated that an amplitude of the vibration of the sprung member at the above resonance points can be reduced to improve driving comfort by increasing a damping force of the suspension means (the damping coefficient c1 of the damper).
However, when the damping force of the suspension means is increased, a stiff shock is transmitted to the vehicle body at any points other than the resonance points, thus rather reducing driving comfort.
To cope with this problem, there has been proposed a technique such that the damping force of the suspension means is made variable, and when a frequency of the vibration falls in a spring or unsprung resonance frequency region, the damping force is temporarily increased to thereby effectively suppress the vibration of the vehicle body throughout the whole frequency region (cf. Japanese Patent Laid-open Publication No. 59-227515).
Generally, damping of the sprung resonance depends on a damping coefficient ratio .zeta.=c1/2 m1k1, and the more the damping coefficient ratio approaches 1, the more the sprung resonance is suppressed. However, the damping coefficient ration is normally set to 0.2-0.3 in consideration of driving comfort in any regions other than the resonance frequency.
When the sprung weight m1 is increased by an increase in passengers or luggage, the damping coefficient ration .zeta. is relatively reduced. As a result, the damping of the sprung member becomes weak to increase the duration of the vibration and accordingly reduce vehicle's driving comfort.
Although the sprung resonance can be suppressed by detecting the same and temporarily increasing the damping force to increase the damping coefficient ratio, the sprung resonance is easily generated when an original damping force is restored.